1. Art Field
The present invention relates to organic/inorganic hybrids, and a process of producing them.
2. Background Art
Hydroxide minerals represented by brucite, portlandite, gibbsite, boehmite, lepidocrocite, etc. are ubiquitous substances that occur abundantly in the surface layer of the earth.
Hydroxide minerals have a variety of features inclusive of absorption capability, catalytic activity, and harmless to the human body (free from antigen-antibody reactions). For this reason, they have been used in a variety of fields in the form of purification adsorbents for water and waste oils, catalysts for petroleum cracking, agents form removal of sulfur oxides from thermal power plant emissions, counteragents for acidified rivers and soils, pharmaceutical raw materials, flame retarders for plastics, and so on.
Now they attract attention as pharmaceutical raw materials and retarders in particular.
For instance, Patent Publication 1 discloses pharmaceutical raw materials harnessing hydroxide minerals excelling in antacid capability, and Patent Publication 2 discloses materials for drug delivery systems (DDS for short) making use of hydroxide minerals. Note here that DDS is a technology for quantitative, spatial and time control of the distribution of biomolecules and drugs injected into the living body.
A typical hydroxide mineral is a multilayered structure having as a basic structure unit layers wherein octahedrons having OH− or O2− hexacoordinating to a metal such as Mg2+, Ca2+, Al3+, and Fe3+ are lined up through edge sharing in a planar configuration. If an organic compound is inserted between the unit layers, its affinity for that organic compound could grow high.
In practical applications of inorganic compounds having a laminated structure like clay minerals, layered titanates, chalcogen compounds, and aluminum-magnesium composite hydroxides (hereinafter called hydrotalcite), an organic compound is inserted between the unit layers instead of interlayer ions (hereinafter also called exchangeable ions) to form a structure (hereinafter called the organic/inorganic hybrid) where the organic compound is hybridized with the inorganic compound in a nanoarea (hereinafter called the nanohybridization). Especially in the development of DDS going deep into the nanoarea, there is an organic/inorganic hybrid formed in which biomolecules or drugs having exchangeable ions are included (or intercalated) between the unit layers.
Non-Patent Publications 1 and 2 show organic/inorganic hybrids prepared by high-temperature treatment of glycols having two carbon atoms or glycerols having three carbon atoms, all being sugar alcohols that are liquid at room temperature.
However, the unit layers of the hydroxide mineral is electrically neutral, and do not have any interlayer ions for neutralizing their charges, with the adjacent planes of the unit layers being strongly bonded to each other by way of hydrogen bonds. It is still difficult to include the organic compound in a narrow space between the unit layers.
There is also still no effective method of cutting hydrogen bonds by way of which the unit layers are bonded together, and it is still impossible to include the organic molecules between the unit layers either.
With the possibility of using hydroxide minerals as DDS materials in consideration, many organic compounds having medication efficacies have a large number of carbon atoms, and it is still difficult to include an organic compound having a lot of carbon atoms in a narrow space between the unit layers.
There has also been difficulty achieving a material that has a sustained release feature good enough to release the included organic compound slowly in constant amounts over the elapse of constant times.
In addition, the aforesaid hydrotalcite contains aluminum, and as it is used as the DDS material, there is a risk of inducing aluminum-associated encephalosis or the like.
It has also been known in the art that hydroxide minerals are usable as flame retarders for plastics.
For instance, brucite and gibbsite dehydrate and decompose drastically at 200 to 400° C., causing an endothermic reaction. With gibbsite, an endothermic reaction of about 1.5 kJ/g keeps going on, and with brucite, an endothermic reaction of 1.39 kJ/g keeps going on. An exothermic reaction of polyethylene is 43.4 kJ per gram. If polyethylene is blended with 66% by mass of gibbsite, the generation of heat by burning is almost leveled with the absorption of heat by the decomposition of gibbsite, resulting in a lowering of the temperature of a burning reaction field, which prevents burning from going on. Hydroxide minerals hinder a burning reaction of plastics, keeps temperature against rising upon burning of plastics, have a self-extinguishing feature, and facilitate prevention of smoking. Furthermore, brucite and gibbsite do not give rise to dioxin or other toxic gases upon burning; they are used as eco-friendly non-halogen flame retarders.
Upon high-density packaging of hydroxide minerals in high molecules such as polyethylene, however, there is a risk of being noticeably detrimental to flexibility and moldability, which offers a grave practical challenging problem.
If an inorganic compound is pulverized down to a nanometer level for dispersion throughout a plastic or rubber component, it is then possible to obtain a nanocomposite material having high flame retardancy. The finely pulverized inorganic compound forms a carbide film called a char on a burning surface, which keeps oxygen against being fed under burning reactions, producing a flame retarding effect.
Even though hydrophilic hydroxide minerals are hybridized into high molecules as they stand, however, it is much detrimental to dispersibility, encountering difficulty obtaining flame retarding features through fine dispersion.